In recent years and in many countries of the world increasing research and experimental work has been performed to find new, effective and economical displacing media in which the secondary exploitation of oil reservoirs may be performed and to increase the efficiency of recovery, the areal and volumetric sweep efficiency, and the recovery factor respectively.
A number of attempts have involved the use of carbon dioxide under pressure. Carbon dioxide, above a certain pressure level, is readily soluble in water, excellently soluble in oil and in the context of the present invention has the following effects:
1. Upon dissolution in the oil it increases the volume of the oil and thus leads to an increased relative oil saturation, to increased effectiveness of displacement of the oil and to increased recovery factor.
2. It decreases the viscosity of the oil, thereby increasing the effectiveness of the oil displacement and the efficiency of recovery, respectively.
3. It decreases the surface and interfacial tension, thereby increasing the effectiveness of displacement of the oil and the recovery factor, respectively.
4. It dissolves calcium carbonate binding material in sandstone and the rock material in limestone reservoirs, thus increasing primarily the permeability of the formation. Further it causes cross-flow and thus leads to increased surface and volumetric effectiveness of displacement and the recovery factor, respectively.
Whorton et al (U.S. Pat. No. 2,623,596) discloses the effect of the percentage of CO.sub.2 in a gas injected during secondary exploitation of an oil reservoir and the injection pressure on the percentage of oil recovered. In a series of tests with CO.sub.2 /N.sub.2 mixtures,it was determined that the minimum amount of CO.sub.2 in the mixture should be 50% and the minimum injection pressure 1,000 psi; preferably followed by the injection of an inert fluid. The use of 100% of CO.sub.2 is preferred but, if the availability of CO.sub.2 is limited, it is disclosed that other gases such as nitrogen, air, natural gas or any other gas can be mixed in. Whorton et al do not indicate any preference for one gas over another.
The effect of CO.sub.2 is in close correlation with the dissolution. Pure carbon dioxide is more soluble in oil, than the so-called mixed gas containing CO.sub.2 and a CH (hydrocarbon) component as well.
Under reservoir conditions, however, it is rare that either the oil or the dissolving gas is free of hydrocarbon gases. Under such conditions, the oil always contains more or less hydrocarbon gas dissolved therein or dissolved in the free gaseous phase. Naturally occurring carbon dioxide gas is what is called a mixed natural gas, containing hydrocarbons. Even if hydrocarbons are removed from the mixed natural gas prior to utilization of the carbon dioxide for secondary recovery of oil reservoirs, there will again be an admixture of hydrocarbon gas with the carbon dioxide upon injection of the carbon dioxide into the reservoir due to the CH components present in the reservoir. The solubility of carbon dioxide in oil is decreased by the presence of these hydrocarbon gases.
In view of the negative effect of hydrocarbon gases on the solubility of CO.sub.2 in oil, which is directly related to the oil recovery percentage, as shown below, it might be deduced that the use of N.sub.2 mixed with CO.sub.2 would be preferable to natural gas as a diluent, if a diluted CO.sub.2 was desired for injection into the oil reservoirs for secondary exploitation. Therefore, it was surprising to discover that, on the contrary, hydrocarbon gases are superior to nitrogen as a diluent in such applications.